1.Field of the Invention
This invention relates to drilling of long generally horizontal holes in a coal seam ahead of mining of the coal to release methane gas from the coal seam. It is desirable to drill such long holes for substantial distances, such as several hundred meters, and the primary problem in such drilling is that of keeping the drill bit in the coal seam. A considerable amount of work has been done by the U.S. Bureau of Mines and private industry in developing techniques controlling the trajectory of long holes. Variables such as bit rotational speed, thrust and stabilizer placement affect the trajectory of a long hole, and directional survey tools are available to enable an operator to check the trajectory of a hole.
There are occasions when it is desirable for an operator to know where in the coal seam the long hole is relative to the roof or floor of the coal seam so that the variables controlling hole trajectory mentioned above can be adjusted to keep the bit in the coal seam.
The present invention provides an apparatus enabling an operator to determine the proximity of a long hole to the roof or floor of a coal seam at any point along the long hole.
2. Description of the Prior Art
A large number of prior art devices are available for drilling directional bores from an existing bore. Most of these devices are intended for directional drilling of oil wells. U.S. Pat. Nos. 2,743,082; 2,669,429 and 2,708,099 are representative of this type device.
Devices for drilling at right angles to a borehole utilizing flexible shafts in guide channels are illustrated in U.S. Pat. Nos. 1,595,922 and 4,051,908. U.S. Pat. No. 1,595,922 purports to provide a means for maintaining the direction of the deviated hole straight out from the primary bore.